Conflict detection and resolution using predicted aircraft trajectories

ABSTRACT

A method of detecting conflicts between aircraft passing through managed airspace, and resolving the detected conflicts strategically. The method may include obtaining intended trajectories of aircraft through the airspace, detecting conflicts in the intended trajectories, forming a set of the conflicted aircraft, calculating one or more revised trajectories for the conflicted aircraft such that the conflicts are resolved, and advising the conflicted aircraft subject to revised trajectories of the revised trajectories.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to and incorporates herein by reference inits entirety, co-pending U.S. patent application Ser. No. 13/902,672,concurrently filed and entitled “Conflict Detection and Resolution UsingPredicted Aircraft Trajectories” which claims priority to EP PatentApplication No. 12382207.4, filed on May 25, 2012.

This application is related to and incorporates herein by reference inits entirety, co-pending U.S. patent application Ser. No. 13/902,568,concurrently filed and entitled “Conflict Detection and Resolution UsingPredicted Aircraft Trajectories” which claims priority to EP PatentApplication No. 12382206.6, filed on May 25, 2012.

This application is related to and incorporates herein by reference inits entirety, co-pending U.S. patent application Ser. No. 13/902,632,concurrently filed and entitled “Conflict Detection and Resolution UsingPredicted Aircraft Trajectories” which claims priority to EP PatentApplication No. 12382209.0, filed on May 25, 2012.

PRIORITY STATEMENT

This application claims the benefit of EP Patent Application No.12382208.2, filed on May 25, 2012 in the Spanish Patent Office, thedisclosure of which is incorporated herein by reference in its entirety.

FIELD

This disclosure relates to automating the management of airspace. Inparticular, the present disclosure is concerned with detecting conflictsbetween aircraft passing through managed airspace, and to resolving thedetected conflicts strategically.

BACKGROUND

Air traffic management is responsible for the safe passage of aircraftthrough an airspace. The aircraft may be manned or unmanned. To do this,a centralised, ground-based air traffic management facility mustcommunicate with aircraft flying through the airspace it manages. Thistwo-way communication may be done in a number of ways, including by oralcommunication such as by radio or by data communication through a datalink or the like.

The aircraft may determine their desired flight path through theairspace, for example using an airborne flight management system, andmay then communicate this to air traffic management. In modern times,air traffic management uses sophisticated computer systems to check thesubmitted flight paths do not result in aircraft trajectories that giverise to conflicts. Conflicts between aircraft arise when their intendedtrajectories would result in a separation falling below the minimumspecified. By trajectory, a four-dimensional description of theaircraft's path is meant such as a time-ordered sequence of aircraftstates, including position and altitude. Maintaining safe separations isa particularly demanding task, particularly in congested airspace suchas around airports where flight paths tend to converge.

In addition to detecting conflicts, air traffic management must have themeans to be able to resolve the conflicts and to communicate thenecessary changes in trajectories to the conflicted aircraft.

To date, most efforts aimed at air traffic management's ability todetect and resolve air traffic conflicts have focused on crossingtraffic patterns and have not dealt with the more challenging problem ofconverging traffic. This arises, for example, in arrivals management atTRACON (terminal radar control) facilities, where aircraft arrive frommany directions and must be sequenced for approach and landing at anairport. The efforts directed to converging traffic consider maximizingthe throughput of traffic on an airspace resource such as a sector or arunway as the main or sole objective when solving air traffic conflicts.Existing solutions also focus on planning the arrival sequence firstbefore detecting and resolving conflicts. The method then proceeds byextrapolating that sequence backwards to the earlier waypoints. However,such an approach only serves to propagate the delay backwards to allother aircraft.

Previous attempts at detecting and resolving conflicts suffer otherproblems. For example, previous attempts have analysed conflicts inisolation from each other, typically as isolated events between pairs ofaircraft. The detected conflicts are resolved in a sequential mannerwithout any consideration of the possibility of a “domino effect”feeding back delays.

Recent advances in predicting aircraft trajectories accurately are ofbenefit to air traffic management. In particular, work on expressingaircraft intent using formal languages provides a common platform forthe exchange of flight information and allows different interestedparties to perform trajectory calculations. For example, this aids thecommunication of planned trajectories between aircraft and air trafficmanagement.

EP patent application 07380259.7, published as EP-A-2,040,137, also inthe name of The Boeing Company, describes the concept of aircraft intentin more detail, and the disclosure of this application is incorporatedherein in its entirety by reference. In essence, aircraft intent is anexpression of the intent of how the aircraft is to be flown. Theaircraft intent is expressed using a set of parameters presented so asto allow equations of motion governing the aircraft's flight to besolved. The theory of formal languages may be used to implement thisformulation. An aircraft intent description language provides the set ofinstructions and the rules that govern the allowable combinations thatexpress the aircraft intent, and so allow a prediction of the aircrafttrajectory.

Flight intent may be provided as an input to an intent generationinfrastructure. The intent generation infrastructure may be airborne onan aircraft or it may be land-based such as an air traffic managementfacility. The intent generation infrastructure determines aircraftintent using the unambiguous instructions provided by the flight intentand other inputs to ensure a set of instructions is provided that willallow an unambiguous trajectory to be calculated. Other inputs mayinclude preferred operational strategies such as preferences withrespect to loads (both payload and fuel), how to react to meteorologicalconditions, preferences for minimising time of flight or cost of flight,maintenance costs, and environmental impact. In addition, other inputsmay include constraints on use of airspace to be traversed.

The aircraft intent output by the intent generation infrastructure maybe used as an input to a trajectory computation infrastructure. Thetrajectory computation infrastructure may be either located with or awayfrom the intent generation infrastructure. The trajectory computationinfrastructure may comprise a trajectory engine that calculates anunambiguous trajectory using the aircraft intent and other inputs thatare required to solve the equations of motion of the aircraft. The otherinputs may include data provided by an aircraft performance model and anEarth model. The aircraft performance model provides the values of theaircraft performance aspects required by the trajectory engine tointegrate the equations of motion. The Earth model provides informationrelating to environmental conditions, such as the state of theatmosphere, weather conditions, gravity and magnetic variation.

SUMMARY

Against this background, and from a first aspect, the present disclosureresides in a computer-implemented method of managing airspace throughwhich a plurality of aircraft are flying.

The method comprises receiving, from the aircraft, user preferredaircraft intent data that unambiguously defines the user preferredtrajectory of each aircraft. The user-preferred aircraft intent data maybe a description of the aircraft's user-preferred trajectory expressedin a formal language or may be a full description of how the aircraft isto be operated expressed in a formal language that may be used tocalculate a corresponding unique trajectory. The description shouldclose all degrees of freedom of motion of the aircraft, and shouldcompletely define the configuration of the aircraft (e.g. flaps, speedbrakes, undercarriage).

The method further comprises calling an initial global conflictdetection procedure. This procedure comprises calculating thecorresponding user preferred trajectories from the user preferredaircraft intent data and comparing the user preferred trajectories so asto identify one or more conflicts between trajectories. The comparisonof the user preferred trajectories is used to identify conflictedaircraft and to place the conflicted aircraft into conflict dependentnetworks such that each conflict dependent network contains a set of allaircraft in conflict with each other, either as a directly linkedconflict or an indirectly linked conflict.

By indirectly linked conflicts, it is meant pairs of aircraft that arelinked via a chain of conflicts. For example, aircraft A and aircraft Dare indirectly linked through conflicts if aircraft A is in conflictwith aircraft B, aircraft B is in conflict with aircraft C, and aircraftC is in conflict with aircraft D. In this case, aircraft A, B, C and Dwould all be placed in the same conflict dependent network.

Each conflict dependent network may be populated with some or all of theconflicted aircraft in the airspace in question. A conflicted aircraftmay be put into a conflict dependent network, along with otherconflicted aircraft that will be in conflict with the first aircraft putinto the conflict dependent network. All aircraft that will be inconflict with the first aircraft may be put into the conflict dependentnetwork. This process may then be repeated for all conflicted aircraftin the airspace, i.e. for a new aircraft to be added to an existingconflict dependent network, it must be in conflict with at least one ofthe aircraft that are already in that network. If the current aircraftis in conflict with one or more aircraft, but none of those aircraft arein an existing conflict dependent network, the current aircraft is putin a new network, along with the aircraft with which it is in conflict.This continues until all conflicted aircraft are in a conflict dependentnetwork.

Thus, as explained above, each network contains all conflicted aircraftthat have either a direct or indirect conflict with other conflictedaircraft in the conflict dependent network. Consequently, a conflictedaircraft can only be a member of one and only one conflict dependentnetwork, i.e. the conflict dependent networks are disjoint(non-overlapping) and cover the entire set of conflicted aircraft in theairspace under consideration.

The method further comprises processing the conflict dependent networksin turn and, for each conflict dependent network, calling an initialconflict resolution procedure. This resolution procedure comprisesrevising the user preferred aircraft intent data of one or more of theconflicted aircraft of the conflict dependent network to produce revisedaircraft intent data that will unambiguously define a correspondingrevised trajectory in a way that should remove conflicts from withinthat conflict dependent network. When all conflict dependent networkshave been processed in this way, the revised aircraft intent data issent to the corresponding conflicted aircraft. Thus, conflicts are byconsidering one conflict dependent network at a time.

Optionally, the method further comprises, during processing of eachconflict dependent network, calling a local conflict detectionprocedure. This procedure may comprise calculating the correspondingrevised trajectories from the revised aircraft intent data and comparingthe user-preferred trajectories of aircraft in the conflict dependentnetwork not subject to revised aircraft intent data and revisedtrajectories so as to identify one or more conflicts betweentrajectories. If conflicts are identified, the comparison of thetrajectories may be used to identify the still-conflicted aircraft andto call a further conflict resolution procedure. The further conflictresolution procedure may comprise revising the user-preferred aircraftintent data of one or more of the still-conflicted aircraft of theconflict dependent network to produce revised aircraft intent data in away that should remove conflicts from within that conflict dependentnetwork, and calling a further local conflict detection procedure.

Thus, the method loops though iterations of local conflict resolutionand detection procedures until all conflicts are removed from theconflict dependent network. The terms “local” conflict resolution anddetection procedures are used to emphasise the fact that conflicts areprocessed at the conflict dependent level at this stage.

When all conflicts are removed from the conflict dependent networkcurrently being processed, then the local conflict detection procedurewill return a “no conflicts found” result, in which case the method maycomprise either continuing to process the next conflict dependentnetwork or, if all conflict dependent networks have been processed,calling a further global conflict resolution procedure.

The further global conflict detection procedure processes all aircraftin the airspace and so considers all conflict dependent networkstogether. The procedure may comprise calculating the correspondingrevised trajectories from the revised aircraft intent data, andcomparing the user-preferred trajectories from the aircraft not subjectto revised aircraft intent data and revised trajectories from theaircraft subject to revised aircraft intent data so as to identify oneor more conflicts between trajectories. If conflicts are detected, themethod may comprise calling the local conflict resolution procedure.Optionally, conflicts between aircraft from separate conflict dependentnetworks maybe identified and those conflict dependent networks may bemerged prior to calling a new round of local conflict procedures.

Thus, once all conflict dependent networks are found to be conflictfree, a global check is made to ensure that revisions to aircraft intentdata has not caused aircraft from different conflict dependent networksto come into conflict with each other. If new conflicts are created,then another round of local conflict resolution and detection proceduresare launched. As revisions to aircraft intent data may include a randomelement, the next round may give rise to a solution that removes allconflicts. As noted above, where new conflicts arise, the conflictedconflict dependent networks may be merged into a new single conflictdependent network. The local conflict resolution and detectionprocedures may then operate on the revised list of conflict dependentnetworks.

The above methods see all conflicts resolved in each conflict dependentnetwork before processing the aircraft intent data globally. However, insome embodiments this need not be the case. For example, if a conflictis found during the first iteration of the local conflict detectionprocedure, the method may not call the further conflict resolutionprocedure but may simply proceed to process the next conflict dependentnetwork. When all conflict dependent networks have been processed, themethod may continue to the global conflict detection procedure. Thisprocedure will identify all conflicts remaining within conflictdependent networks and also any new conflicts between different conflictdependent networks, and will then call a new round of local conflictresolution procedures where the conflicts can be addressed.

If no conflicts are detected during the global conflict detectionprocedure, the method may continue to the step of sending the revisedaircraft intent data to the corresponding conflicted aircraft.

Optionally, performing the initial conflict resolution procedure and anyfurther conflict resolution procedures comprises taking the conflictdependent networks in turn in increasing order of the lateness of theearliest occurring conflict within the conflict dependent network. Thetime of the conflict may be taken as the time that the aircraft firstcome into conflict, e.g. their separation drops below a minimum allowed.The network with the first occurring conflict may be processed first andthen removed from the list of networks to be processed. Then the networkfrom the revised list with the earliest occurring conflict may beprocessed, and so on.

Performing the initial conflict resolution procedure or any step ofperforming the further conflict resolution procedure may comprise takingthe conflict dependent networks in turn and for each conflict dependentnetwork, selecting one or more identified conflicts from the conflictdependent network, characterising the selected conflict, andresponsively revising the user preferred aircraft intent data of one ofthe aircraft involved in the selected conflict in a way that shouldremove the conflict.

Characterising the selected conflict may comprise determining the causeof the reduced separation between the conflicted aircraft, and revisingthe aircraft intent data comprises revising aircraft intent data toreverse the cause.

Revising the aircraft intent data may cause at least one of: a change inaltitude of one of the conflicted aircraft, a change in speed of one ofthe conflicted aircraft, or a change in path length of one of theconflicted aircraft optionally by adding or removing waypoints from thepath.

Accordingly, the step of responsively revising the user-preferredaircraft intent data or revised aircraft intent data may be performed byselecting a revision from a set of candidate resolution patterns. Thestep of responsively revising the user-preferred aircraft intent data orrevised aircraft intent data may further comprise storing each instanceof revised aircraft intent data so as to form a first joint candidateresolution pattern when all conflicts have been removed. The, the stepsof calling the initial conflict detection and resolution proceduresmaybe repeated and, if conflicts are found, the further conflictdetection procedure may be called so as to form at least a second jointcandidate resolution pattern. The method may further comprise selectingone of the joint candidate resolution strategies and sending the revisedaircraft intent data from the selected joint candidate resolutionstrategy to the corresponding conflicted aircraft.

The method may comprise selecting one of the joint candidate resolutionstrategies according to an evaluation of the total change in path lengthor time of arrival of the revised trajectories for each joint candidateresolution strategy. The method may comprise selecting one of the jointcandidate resolution strategies according to an evaluation of how eachjoint candidate resolution strategy distributes changes in trajectoriesbetween the aircraft or changes in time of arrival between the aircraft.The step of characterising the selected conflict in the initial orfurther conflict resolution procedure may comprise determining the causeof the reduced separation between the conflicted aircraft, and revisingthe aircraft intent data comprises revising aircraft intent data toreverse the cause.

The candidate resolution patterns may include patterns that cause theaircraft intent data to be revised to cause at least one of: an increasein altitude of one of the conflicted aircraft, a decrease in altitude ofone of the conflicted aircraft, an increase in speed of one of theconflicted aircraft, a decrease in speed of one of the conflictedaircraft, an increase in path length of one of the conflicted aircraftoptionally by adding one or more waypoints to the path, and a decreasein path length of one of the conflicted aircraft optionally by removingone or more waypoints from the path.

The step of responsively revising the user preferred aircraft intentdata or revised aircraft intent data in the initial or further conflictresolution procedure may be performed in a partially random manner. Forexample, selecting a revision from a set of candidate resolutionpatterns may comprise determining which of the patterns are suitable forremoving the selected conflict, and selecting randomly one of thesuitable patterns. Optionally, selecting a revision from a set ofcandidate resolution patterns may comprise determining which of thepatterns are suitable for removing the selected conflict, selecting oneof the suitable patterns (optionally, in a random manner) and revising aparameter associated with the selected pattern by a random amount.

There is also provided a further computer-implemented method of managingairspace through which a plurality of aircraft are flying.

The method comprises receiving, from the aircraft, user preferredaircraft intent data that unambiguously defines the user preferredtrajectory of each aircraft. The user-preferred aircraft intent data maybe a description of the aircraft's user-preferred trajectory expressedin a formal language or may be a full description of how the aircraft isto be operated expressed in a formal language that may be used tocalculate a corresponding unique trajectory. The description shouldclose all degrees of freedom of motion of the aircraft, and shouldcompletely define the configuration of the aircraft (e.g. flaps, speedbrakes, undercarriage).

The method further comprises calling an initial conflict detectionprocedure. This procedure comprises calculating the corresponding userpreferred trajectories from the user preferred aircraft intent data. Theuser preferred trajectories are compared so as to identify one or moreconflicts between trajectories and to identify conflicted aircraftpredicted to fly the identified conflicting trajectories. In someembodiments of the invention, there are more than one conflict detectionprocedure, but in other embodiments there is only a single conflictdetection procedure. This, the term “initial conflict detectionprocedure” should be interpreted to cover embodiments comprising asingle conflict detection procedure.

The method then comprises calling an initial conflict resolutionprocedure. This conflict resolution procedure comprises selecting one ormore identified conflicts, characterising the selected conflict andresponsively revising the user preferred aircraft intent data of one ofthe aircraft involved in the selected conflict in a way that shouldremove the conflict. Examples of how the aircraft intent data may berevised responsively are given below. Essentially, the act ofcharacterising a conflict may include determining if one aircraft is afollowing aircraft that is approaching another, leading aircraft, andrevising the aircraft intent data may involve ensuring that the leadingaircraft stays safely ahead of following aircraft by adjusting theaircraft intent data of the leading aircraft and/or the followingaircraft. For example, the speed of the leading aircraft may beincreased or the path of the following aircraft may be lengthened.Although described as the “initial conflict resolution procedure”, itmay be the only conflict resolution procedure: the term “initialconflict resolution procedure” is used to assist in describing someembodiments of the present invention that have more than one conflictresolution procedure.

The method also comprises sending the revised aircraft intent data tothe corresponding conflicted aircraft.

The above method sees aircraft intent data revised to remove conflicts.This may be done in a targeted way to ensure that the conflicts areremoved. However, in some embodiments it is preferred to check that allconflicts have been removed. For example, all conflicts may be selectedand the user-preferred aircraft intent data of at least one of theconflicted aircraft revised to remove the conflict.

Accordingly, the method may further comprise calling a further conflictdetection procedure. This further conflict detection procedure maycomprise calculating the corresponding revised trajectories from therevised aircraft intent data, and comparing the user-preferredtrajectories from aircraft not subject to revised aircraft intent dataand revised trajectories so as to identify one or more conflicts betweentrajectories. If conflicts are found, the method may compriseidentifying still-conflicted aircraft predicted to fly the identifiedconflicting trajectories.

Also, if conflicts are identified during the further conflict detectionprocedure, the method may comprise calling a further conflict resolutionprocedure comprising selecting one or more identified conflicts,characterising the selected conflict and responsively revising the userpreferred aircraft intent data or revised aircraft intent data of one ofthe aircraft involved in the selected conflict in a way that shouldremove the conflict. The method may then comprise calling the furtherconflict detection procedure.

If no conflicts are detected, the method may comprise continuing to thestep of sending revised aircraft intent data.

Thus, a loop of further conflict resolution and further detectionprocedures is defined that may be repeated until the user preferred andrevised trajectories are conflict free. This loop may be terminated, forexample if it is determined that a conflict free set of aircraft intentdata cannot be generated. When a further conflict detection procedurefails to identify any conflicts, the revised aircraft intent data may besent to the corresponding conflicted aircraft.

Characterising the selected conflict may comprise determining the causeof the reduced separation between the conflicted aircraft, and revisingthe aircraft intent data comprises revising aircraft intent data toreverse the cause. For example, if an aircraft is found to be catchingup with a preceding aircraft, the following aircraft may have itsaircraft intent data revised to cause its speed to decrease.Alternatively, an aircraft found to be descending towards anotheraircraft may have its aircraft intent data revised to hold an altitude.

Revising the aircraft intent data may causes at least one of: a changein altitude of one of the conflicted aircraft, a change in speed of oneof the conflicted aircraft, or a change in path length of one of theconflicted aircraft optionally by adding or removing one or morewaypoints from the path.

Revising the aircraft intent data may be performed by selecting arevision from a set of candidate resolution patterns. For example, acandidate resolution pattern may be selected that is described astargeting a particular characteristic of conflicts. For example,candidate resolution patterns describing an increase in speed orremoving waypoints may be described as targeting an aircraft that isbeing caught up by the following aircraft. As there may be more than onesuitable candidate resolution pattern, one of the suitable patterns maybe selected at random.

While the selection of candidate resolution patterns may be targeted toreverse the cause of a conflict, the magnitude of the change may berandomly selected. For example, a revision may be made to increasespeed, although the magnitude of the change in speed may be randomlygenerated (optionally, randomly generated within limits e.g. safeoperating speeds of the aircraft).

Optionally, the step of responsively revising the user preferredaircraft intent data or revised aircraft intent data in the initial orfurther conflict resolution procedure further may comprise storing eachinstance of revised aircraft intent data so as to form a first jointcandidate resolution pattern when all conflicts have been removed. Themethod may then further comprise repeating the steps of calling theinitial conflict detection and resolution procedures and, if conflictsare found, calling the further conflict detection procedure so as toform at least a second joint candidate resolution pattern. Then, one ofthe joint candidate resolution strategies may be selected, and therevised aircraft intent data from the selected joint candidateresolution strategy maybe sent to the corresponding conflicted aircraft.

Different criteria may be used for selecting one joint candidateresolution strategy over another. For example, selection may be madedependent upon an evaluation of the total change in path length or timeof arrival of the revised trajectories for each joint candidateresolution strategy. Another possibility is to select one of the jointcandidate resolution strategies according to an evaluation of how eachjoint candidate resolution strategy distributes changes in trajectoriesbetween the aircraft or changes in time of arrival between the aircraft.The strategy that distributes revisions most equally or most fairly maybe chosen. This allows strategies that revises moderately manytrajectories to be favoured over strategies that revise only one or afew trajectories but that revise them considerably.

Hence, a candidate strategy may be chosen dependent upon cost, forexample using a mathematical routine using cost functions. For example,the method may comprise calculating the cost of the revisedtrajectories. The cost may be a measure of how much each revisedtrajectory differs from the corresponding user-preferred trajectory, forexample as a time cost or a distance cost. Hence, choosing a candidatestrategy may be based on determining the candidate strategy that spreadsthe costs most evenly amongst the conflicted aircraft, for example byfairness or equity. A candidate strategy may be chosen that cannot bealtered such that one conflicted aircraft's costs decrease withoutincreasing the cost of another conflicted aircraft.

The present disclosure also extends to a computer apparatus programmedto implement any of the methods described above. The present disclosurealso extends to a computer program comprising instructions that whenexecuted on a computer cause the computer to perform any of the methodsdescribed above, and to a computer readable medium having stored thereonsuch a computer program. The present disclosure also extends to an airtraffic control apparatus comprising a computer apparatus programmed toimplement any of the methods described above.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present disclosure may be more readily understood,preferred embodiments will now be described, by way of example only,with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram showing aircraft flying within an airspacemanaged by an air traffic management facility;

FIG. 2 shows a framework illustrating the relationship between airtraffic management and an aircraft flying within the airspace it managesthat allows conflict detection and resolution;

FIG. 3 is a schematic representation of a negotiation process between anaircraft an air traffic management;

FIG. 4 is a flow chart representation of a method of detecting andresolving conflicts according to an embodiment of the present invention;

FIG. 5 is a flow chart representation of a method of detecting andresolving conflicts according to another embodiment of the presentinvention;

FIG. 6 is a flow chart representation of a system for detecting andresolving conflicts according to an embodiment of the present invention;

FIG. 7 is a flow chart representation of a conflict detection process;

FIGS. 8 a and 8 b show two examples of conflicting trajectories;

FIGS. 9 a and 9 b show two examples of how trajectories may be modifiedto resolve conflicts; and

FIG. 10 is a flow chart representation of a conflict resolution process.

DETAILED DESCRIPTION

The present disclosure provides methods and systems that enable aground-based airspace management system to de-conflict strategically thetrajectories of aircraft under its responsibility, regardless of whetherthe aircraft are manned or unmanned, in any traffic scenario includingconverging traffic patterns.

System Overview

FIG. 1 shows schematically an airspace 10 under the control of airtraffic management facility 12. In this example, air traffic management12 is located at an airport 14 and is responsible for aircraft 16arriving and departing from the airport 14, as well as those aircraft 16passing through the airspace 10.

Air traffic management 12 is provided with associated communicationmeans 18 to allow two-way communication with the aircraft 16 flyingthrough the airspace 10. The aircraft 16 are equipped with complementarycommunication equipment (not shown in FIG. 1) of any type well known inthe field of aerospace. For example, communication may be effected byradio or could be effected using a data link such as ADS-B.

Communication between air traffic management 12 and each of the aircraft16 is generally the same, and may be effected either in parallel orserially. A framework illustrating the relationship between air trafficmanagement 12 and one of the aircraft 16 will now be described in moredetail. It is to be understood that this framework is common to all theaircraft in the sense that it is the same for any aircraft 16 chosen tobe considered.

FIG. 2 shows schematically the airborne system 20, the ground-basedsystem 22, and the negotiation process 24 that occurs between theairborne system 20 and ground-based system 22. Airborne system 20 isprovided by the aircraft 16, and the ground-based system 22 is providedby air traffic management 12. The negotiation process 24 requires acommunication system 26 that is distributed between the aircraft 16 andair traffic management 12, namely a transmitter/receiver provided on theaircraft 16 and the communication means 18 provided at the air trafficmanagement facility 12.

In the example of FIG. 2, the communication system 26 is used toexchange aircraft intent data 28 between the airborne automation system20 and the ground-based automation system 22. The aircraft intent data28 may be provided by the airborne automation system 20 or by theground-based automation system 22. The aircraft intent data 28 providedby the airborne automation system 20 will correspond to theuser-preferred trajectory of the aircraft 16, whereas the aircraftintent data 28 provided by the ground-based automation system 22 willcorrespond to a revised trajectory determined by air traffic management12.

The airborne automation system 20 comprises flight management logic 30and trajectory computation infrastructure 32. Both these components arecomputer-implemented, preferably as separate computer systems. Forexample, the flight management logic 30 may be part of a flight computerof the aircraft 16.

The flight management logic 30 is responsible for following andsupervising the negotiation process 24 from the aircraft's point ofview. The flight management logic 30 is also responsible for definingthe user-preferred aircraft intent data 28 and agreeing the revisedaircraft intent 28 with the ground-based automation system 22.

The trajectory computation infrastructure 32 is responsible forcomputing the trajectory resulting from a given flight intent 28. Forexample, it may calculate the trajectory arising from a user-preferredaircraft intent for presentation to a pilot for approval before thecorresponding user-preferred aircraft intent data 28 is provided to theground-based automation system 22. Additionally, the trajectorycomputation infrastructure 32 may generate and display a trajectorycorresponding to revised aircraft intent data 28 provided by theground-based automation system 22 such that the pilot may approve therevised trajectory.

The ground-based automation system 22 comprises traffic management logic34 and trajectory computation infrastructure 36. Both these componentsare computer-implemented, preferably as separate computer systems.Although the trajectory computation infrastructure 36 performs a similarfunction to the trajectory computation infrastructure 32 of the airborneautomation system 20, it need not be the same and may be implementeddifferently.

The traffic management logic 34 is responsible for following andsupervising the negotiation process 24. The traffic management logic 34is also responsible for revising aircraft intents where conflicts arise.To enable revision of the aircraft intents, the traffic management logic34 has at its disposal algorithms relating to a look ahead process thatgoverns when to run a conflict detection process, to conflict detectionand to conflict resolution. In this example, the traffic managementlogic 34 is modular in its nature such that any of the algorithms may bevaried or entirely replaced without affecting the other algorithms. Thismodularity also makes the traffic management logic 34 ideal as a testbed for developing improved algorithms in that revised versions of thealgorithm may be readily swapped in and out of the traffic managementlogic 34.

The trajectory computation infrastructure 36 is responsible forgenerating trajectories corresponding to aircraft intents at theground-based automation system 22. The aircraft intent may be theuser-preferred aircraft intent 28 received from the airborne automationsystem 20 or the revised aircraft intent 28 determined by the trafficmanagement logic 34.

The negotiation process 24 defines the type of information to be sharedbetween the airborne automation system 20 and ground-based automationsystem 22. The negotiation process 24 also defines who is to startcommunication according to what events, and the sequence of decisions tobe followed in order to agree upon a revised aircraft intent 28. FIG. 3shows the steps of the negotiation process 24, and will now be describedin more detail.

In this example, the negotiation process 24 starts on-board the aircraft16 with the definition of the aircraft's intent that corresponds to auser-preferred trajectory. This is shown in FIG. 3 at 40. The aircraft16 establishes contact with air traffic management 12 and transmits theuser-preferred trajectory information 42 expressed as the user-preferredaircraft intent data 28 a to air traffic management 12.

Once the user-preferred aircraft intent data 28 a has been received, theaircraft 16 and air traffic management 12 engage in a one-to-onenegotiation process. During the negotiation process 24, theuser-preferred aircraft intent data 28 a submitted by the aircraft 16 isused by the trajectory computation infrastructure 36 to produce thecorresponding trajectory. This user-preferred trajectory is analyzed bythe traffic management logic 34 in order to detect potential conflictswith other aircraft trajectories.

When conflicts are detected, the airborne automation system 20 and theground-based automation system 22 will follow the predeterminednegotiation protocol required by the negotiation process 24 to agree ontrajectory modifications to remove the conflict. The negotiation process24 includes exchange of trajectory information as the aircraft intentdata 28 and, as this is a common characteristic to all possiblenegotiation protocols, it advantageously allows the protocols to beinterchangeable.

Once the user-preferred aircraft intent data 28 a has been received byair traffic management 12 as shown at 44 in FIG. 3, the negotiationprocess 24 continues with a look-ahead process at 46. The look-aheadprocess 46 operates to determine when to launch a conflict detectionprocess 110 and which aircraft (and their trajectories) have to beincluded in that process. Different look-ahead processes 46 may beimplemented as long as pre-established interfaces are maintained.

The look-ahead process 46 may run the conflict detection process 110periodically. The rate of repetition may be varied, for exampleaccording to the volume of air traffic. In addition or as analternative, the conflict detection process 110 may be invoked whenevera new aircraft enters the managed airspace. Further details are givenbelow.

Once the look-ahead process 46 decides which aircraft 16 are going to beincluded in the conflict detection process 110, the conflict detectionprocess 110 is launched. Here, as well, different conflict detectionprocesses 110 may be implemented as long as the pre-establishedinterfaces are maintained. In summary, the conflict detection process110 computes the user-preferred trajectories corresponding to theuser-preferred aircraft intent data 28 a received, and analyses thetrajectories computed to identify potential conflicts. When anyconflicts are identified by the conflict detection process 110, theconflict resolution process 120 is launched.

The conflict resolution process 120 performs calculations to revise theuser-preferred aircraft intent data 28 a to generate revised aircraftintent data 28 b. The revised intents result in corresponding revisionsto the user-preferred trajectories in order to remove the identifiedconflicts. Different conflict detection processes 110 may be implementedas long as the pre-established interfaces are maintained.

As will be explained below, the conflict resolution process 120 callsthe conflict detection process 110 to analyse the revised trajectoriesresulting from the revised aircraft intent data 28 b it proposes toensure that no conflicts remain and that no new conflicts are generated.Once it is confirmed that no conflicts arise, the revised aircraftintent data 28 b are transmitted to the affected aircraft 16 by airtraffic management 12, as shown at 52 in FIG. 3.

The revised aircraft intent data 28 b are received by the aircraft 16under the current consideration, as shown at 54. The aircraft 16 maygenerate a corresponding revised trajectory. In some embodiments, theaircraft 16 is obliged to follow the revised trajectory defined by therevised aircraft intent data 28 b. In other embodiments, including theembodiment currently being described, the aircraft 16 is given theoption of rejecting the revised aircraft intent data 28 b. In this casea further round of negotiation is required or, if time does not allow,the aircraft 16 may be commanded to accept the revised trajectory by theground-based automation system 22. The further round of negotiation maysee a new set of revised aircraft intent data 28 b sent to the aircraft16 for review of the corresponding new revised trajectory. If improvedaircraft intent data 28 b cannot be found, or if computation time forthe negotiation process runs out, the ground-based automation system 22may command the aircraft 16 to follow the original aircraft intent data28 b. In any event, once the revised aircraft intent data 28 b isaccepted and the corresponding trajectory is executed by the aircraft 16as shown at 56. As will be appreciated, the conflict detection andresolution process is a dynamic process, and so further changes may beimposed on the trajectory as it is executed by the aircraft 16.

Conflict Detection and Resolution Overview

Methods of detecting and resolving conflicts in predicted aircrafttrajectories are now described. These methods ensure that the resolvedtrajectories do not result in further conflicts downstream, henceavoiding a “domino effect” of conflicting trajectories propagatingbackwards through the chain of aircraft.

The overall conflict detection and resolution process may be envisagedas a two-stage process of firstly detecting conflicts and secondlyresolving the conflicts. This is illustrated in FIG. 4 by the dashedboxes 110 and 120. Generally, an initial stage of obtaininguser-preferred trajectories of aircraft 16 is performed, as shown bydashed box 100 in FIG. 4. Also, a final stage of advising aircraft 16 ofrevised aircraft intent data 28 b is generally performed, as indicatedby dashed box 130 in FIG. 4. A more detailed description of the fullerfour-stage method of FIG. 4 will now be provided.

The method of FIG. 4 may be practised by a ground-based automationsystem 22 hosted at an air traffic management facility 12, for exampleusing a network of computers located at the facility 12, as describedabove. Air traffic management 12 will assume responsibility for the safepassage of aircraft through the airspace 10 that it manages. The methodstarts at 101 where user-preferred trajectories of the aircraft 16flying through the managed airspace 10 are obtained. This may be done inseveral different ways. For example, a description of the user-preferredtrajectories may be provided. Alternatively, the trajectories may becalculated and hence predicted as part of the method. A description ofan aircraft's user-preferred intent data 28 a may be provided, forexample expressed using a formal language, as shown at 28 in FIG. 2. Airtraffic management 12 may then use this user-preferred aircraft intentdata 28 a to calculate a user-preferred trajectory for the aircraft 16.

With the trajectory prediction process 100 complete, the method moves tothe conflict detection process 110. At step 111, aircraft trajectoriesare compared and conflicts identified. This process is described in moredetail below. At 112, the aircraft 16 predicted to fly conflictingtrajectories are identified and these aircraft are nominally placed intoa set of conflicted aircraft at step 113.

The method then progresses to the conflict resolution process 120. Atstep 121, the set of aircraft formed at step 113 is used. The userpreferred aircraft intent data 28 a of aircraft identified within theset of conflicted aircraft are adjusted and the correspondingtrajectories are calculated to identify one or more instances where allconflicts are resolved.

Once the conflicts are resolved, the method may progress to process 130where conflicted aircraft 16 are advised of their revised aircraftintent data 28 b. This may involve sending a description of theassociated aircraft intent such that the aircraft 16 may then calculatethe corresponding trajectory or it may involve transmitting adescription of the new trajectory to the aircraft 16. The former examplewas described above. As a description of aircraft intent is bydefinition a set of instructions that unambiguously define a trajectory,it is assured that the aircraft 16 will generate the intendedtrajectory.

As will be appreciated, the above method will be performed repeatedly byair traffic management 12. This accounts for variable conditions thatmay otherwise affect the calculated trajectories. For example,unexpected winds may give rise to conflicts that were not previouslypredicted. Repetition of the method may also be used to check thataircraft 16 are indeed following the user-preferred trajectories andthat the airspace remains free of predicted conflicts. Although the rateof repetition may be varied, as an example the method may be repeated atset intervals of every thirty seconds. In addition or as an alternative,the method may be invoked whenever a new aircraft 16 enters the managedairspace 10. As well as including all aircraft 16 within the managedairspace 10, the method may also consider aircraft 16 approaching theairspace 10.

FIG. 5 shows another method of managing an airspace 10, includingdetecting and resolving trajectories of aircraft 16, according to anembodiment of the present invention. According to the embodiment of FIG.2, the method is integrated in a ground-based automation system 22 andworks as follows.

At 102, the traffic management logic 34 of the ground-based automationsystem 22 receives a description of the user-preferred trajectories ofthe aircraft within its area of responsibility. The trajectories aredescribed by the user-preferred aircraft intent data 28 a expressedusing an aircraft intent description language.

At 103, the traffic management logic 34 sends the user-preferredaircraft intent data 28 a to the trajectory computation infrastructure36 that processes those data and predicts the correspondinguser-preferred trajectories.

At 114, possible conflicts are identified, i.e. instances where theseparation between user-preferred trajectories are in violation ofestablished minimum distances between the aircraft 16.

At 115, the detected conflicts are grouped into conflict dependentnetworks. Each network includes all aircraft 16 in conflict with atleast one other aircraft 16 within the network. For example, if aircraftA1 conflicts with aircraft A2, and aircraft A2 conflicts with aircraftA3 and A4 and aircraft A4 conflicts with aircraft A5, a conflictdependent network is formed containing aircraft A1, A2, A3, A4 and A5.All aircraft 16 within the network have conflict dependencies on thetrajectories of all the other aircraft 16 in the network, eitherdirectly or indirectly. A consequence of these types of networks is thatany particular aircraft 16 can be a member of only one conflictdependent network.

At 120, the conflicts are resolved “network-wise”, i.e. consideringsimultaneously all conflicts in a conflict dependent network. In thisway, the implications of the resolution actions on other conflictswithin the network are taken into account from the outset. Theresolution actions are the actions needed to be taken by an aircraft 16to avoid the conflict. These actions are designed as amendments to theuser-preferred aircraft intent data 28 a that produce revisedtrajectories.

As indicated at 122, the resolution actions for the conflicting aircraftwithin a conflict dependent network are selected from a set of jointcandidate resolution strategies (JCRS). The joint candidate resolutionstrategies are derived from a set of predefined joint candidateresolution patterns (JCRP). The selection is carried out so that theselected joint candidate resolution strategy belongs to a set ofPareto-optimal joint candidate resolution strategies. This set of jointcandidate resolution strategies that solve each conflict dependentnetwork are gathered together at step 123. Pareto optimality in thiscontext may be defined in different ways. For example, it may relate tothe changes in flight times or it may relate to a joint cost functioncapturing the additional operating costs, resulting from the resolutionactions as applied across all the aircraft 16 in the conflict dependentnetwork. Thus, the resolution actions in the selected joint candidateresolution strategy are such that the aircraft 16 belonging to the sameconflict dependent network share the consequences of the trajectorymodifications required to resolve the conflicts. For example, thestrategy that sees more, shorter delays spread across more aircraft 16may be preferred to a strategy that sees fewer, larger delays applied toonly a few aircraft 16. At step 124, the optimum joint candidateresolution strategy is selected for each conflict dependent network.

Once the joint candidate resolution strategy has been selected, theaircraft 16 whose trajectories have been amended are identified and therevised aircraft intent data 28 b are communicated to the affectedaircraft 16, as indicated at 132.

In this way, it is possible to solve the problem of resolving airtraffic conflicts strategically in a trajectory-based operationalenvironment by sharing consequences of changes resulting from theresolution of the conflicts among all aircraft involved.

FIG. 6 shows a further embodiment of a ground-based automation system300, that may be used to implement the method of FIG. 4 or FIG. 5. Theground-based automation system 300 comprises three sub-systems, namely atrajectory prediction module 302, a conflict detection module 304 and aconflict resolution module 306.

The ground-based automation system 300 receives as an input adescription of the trajectories of the aircraft expressed asuser-preferred aircraft intent data 28 a using an aircraft intentdescription language (AIDL), as indicated at 301.

The trajectory prediction module 302 calculates the user-preferredtrajectories and provides them as output 303. The user-preferredtrajectories 303 are taken as an input by the conflict detection module304.

The conflict detection module 304 uses the user-preferred trajectoriesto detect conflicts and to group the conflicts into conflict dependentnetworks, as has been described above. The conflict detection module 304provides the conflict dependent networks as an output 305 that isprovided to the conflict resolution module 306.

The conflict resolution module 306 operates on the conflict dependentnetworks to produce joint candidate resolution strategies for eachconflict dependent network, and outputs a joint candidate resolutionstrategy at 307. This joint candidate resolution strategy is used todetermine the data to be sent to affected aircraft by a communicationsystem 308. Although the communication system 308 is shown as beingseparate to the ground-based automation system 300, it may be a part ofthe ground-based automation system 300. For example, the modules 302,304 and 306 and, optionally, the communication system 308 may beprovided as a computer system. The computer system may comprise a singleserver, a plurality of servers and may be provided at a single locationor as part of a distributed network.

As noted above, the two key processes in the method are the conflictdetection process 110 and the conflict resolution process 120. Each ofthese processes will now be described in more detail.

Conflict Detection

FIG. 7 shows the steps involved in a preferred form of the conflictdetection process 110. FIG. 7 shows the process 110 starting at 402. Atstep 404, data is collected. Specifically, a conflict detection (CD)list of aircraft 405 is compiled. The aircraft list 405 to be consideredby the conflict detection process is the list of aircraft 405 known atthe time when the conflict detection and resolution processes arelaunched.

Each aircraft 16 in the aircraft list 405 must have associated certainpieces of information that are required to carry out the conflictdetection process 110. These pieces of information are referred to asconflict detection attributes, and are initially provided together withthe aircraft list 405. The conflict resolution process 120 may in turnalter the conflict detection attributes when subsequently calling theconflict detection process 110 in order to verify whether the revisedaircraft intent data 28 b and the corresponding revised trajectories areindeed conflict free. The main conflict detection attributes aredescribed below.

Type: each aircraft 16 in the list 405 is marked as either available orunavailable, referred to hereinafter as “unlocked” or “locked”. Anaircraft 16 has a preferred trajectory that it would like to fly. Thattrajectory is expressed as the aircraft intent or, in other words, howthe aircraft would like to fly that trajectory. If that intention to flycan still be changed, this means the aircraft 16 and air trafficmanagement 12 have not yet agreed to it, in which case the aircraft isavailable or unlocked. If it cannot be changed, the aircraft 16 isunavailable or locked.

Initial conditions: the available aircraft 16 have associated anestimated time and aircraft state at sector entry (i.e. at the time ofentering the managed airspace 10). These data represent the predictedinitial conditions of the aircraft 16 at sector entry and theseconditions are the starting point for the predictions and search forconflicts.

Current aircraft intent: the current aircraft intent of an unlockedaircraft may be that aircraft's user-preferred aircraft intent 28 a, ora revised aircraft intent 28 b resulting from a previous conflictdetection and resolution process.

At 406, the timeline of the current conflict detection and resolutionprocess is discretized.

Next, at 408, the conflict detection process 110 calls a trajectorypredictor (TP) of the trajectory computation infrastructure 36 topredict the trajectories within its sector for all the aircraft 16 inthe aircraft list 405 from the current simulation time forward. Theinputs to the trajectory computation process are the initial conditionsand the current aircraft intent 28 provided as the aircraft's conflictdetection attributes. This provides the aircraft state at eachprediction time step for all aircraft 16, as indicated at 409.

Once the trajectory predictions are available, the conflict detectionprocess 110 starts calculating the evolution of the inter-aircraftdistances for all possible aircraft pairs along the prediction timeline.In this embodiment, the term inter-aircraft distance refers to theshortest distance over the Earth's surface between the groundprojections of the position of two aircraft 16. Inter-aircraft distanceis used because it is assumed that aircraft 16 must maintain horizontalseparation at all times and that, consequently, the separation minimaapplicable are expressed in terms of inter-aircraft distance, e.g. radarseparation or wake vortex separation. Thus, a conflict occurs when thepredicted inter-aircraft distance between two aircraft 16 falls belowthe applicable minimum during a certain time interval. The conflictdetection process 110 has access to a database containing the applicableminima, which are inter-aircraft distance values that must not beviolated. These minima may depend on the aircraft type, and the relativeposition of the aircraft 16 (e.g. wake vortex separation may prevailbetween aircraft 16 following the same track, but not between aircraft16 on converging tracks). During this process, regard may be paid to thevertical separation of aircraft 16, e.g. to allow reduced horizontalseparation where the vertical separation is sufficient to allow this.

The conflict detection process 110 starts at step 410 where theinter-aircraft distances are calculated for the initial conditions, i.e.the origin of the timeline. Next, at step 412, all possible pairs ofaircraft 16 are formed as shown at 413, and heuristics are applied toeach pair of aircraft 16. At each time step, the conflict detectionprocess 110 applies some heuristics before calculating theinter-aircraft distances, in order to skip aircraft pairs that, giventhe prior evolution of their inter-aircraft distance and their relativepositions, cannot possibly enter into a conflict during the current timestep. In addition, other heuristics will be in place to accelerate thecalculation of the inter-aircraft distances and the comparison with theapplicable minima.

Once the heuristics have been applied, the remaining aircraft pairs havetheir inter-aircraft distances calculated at 414. These inter-aircraftdistances are checked against the applicable separation minima at 416.At 418, the list of conflicts is updated with the newly identifiedconflicts. This step includes creating the new conflicts in the list andupdating associated attributes, as shown at 419.

Once step 418 is complete, the conflict detection process 110 canproceed to the next time step, as shown at 420. A check is made at step422 to ensure that the next time step is not outside the predictionwindow as indicated at 423 (i.e. the conflict detection process willlook forward over a certain time window, and the time steps should moveforward to cover the entire window, but should not go beyond thewindow). Provided another time step is required, the conflict detectionprocess 110 loops back to step 412 where heuristics are applied for thenext time step.

In this way, the conflict detection process 110 proceeds along theprediction time line, from the start to the end of the predictionwindow, calculating the inter-aircraft distance between all possibleaircraft pairs at each time step. The conflict detection process 110 isable to identify all conflicts between the aircraft 16 in the aircraftlist 405 between the start and end of the prediction timeline. Theconflict detection process 110 compiles the identified conflicts into aconflict list, where each conflict is associated with the followingpieces of information, denoted as conflict attributes.

Conflicting aircraft pair: identifiers of the two conflicting aircraft16, together with their conflict detection attributes.

Conflict type: an identifier associated to the type of conflict. In thisparticular embodiment, only two types of conflicts can occur. The firsttype, catching-up conflicts, is shown in FIG. 8 a where the loss ofseparation occurs between aircraft 16 flying along the same track, i.e.their separation dactual falls below the minimum separation alloweddmin. The second type, merging conflicts, is shown in FIG. 8 b where theloss of separation takes place between two aircraft 16 on convergingtracks as they approach the merging point, i.e. their separation dactualfalls below the minimum separation allowed dmin.

Conflict interval: the time interval, in the prediction timeline, duringwhich the inter-aircraft distance is below the applicable minimum.

Conflict duration: the length, in time steps, of conflict interval, i.e.the number of times steps during which the inter-aircraft distance isbelow the applicable minimum.

Conflict intensity: this attribute is a value between 0 and 10 thatprovides a measure of the severity of the conflict (with 0 being thelowest level of severity and 10 the highest). The conflict intensity isa function of the minimum predicted inter-aircraft distance during theconflict and is calculated taking into account the proportion of theapplicable minimum violated by that minimum distance. For example, aminimum predicted separation of 2 miles will result in a conflictintensity of 4.0 when the applicable minimum is 5 miles, and 6.7 whenthe applicable minimum is 3 miles.

Aircraft intent instructions associated with the conflict: the conflictdetection process 110 associates the set of aircraft intent instructionsthat are active for each of the two conflicting aircraft during theconflict interval.

Subsequently, at 424, the identified conflicts are grouped into conflictdependent networks according to an equivalence relation (called theconflict dependency relation) that is defined over the set ofconflicting aircraft 16. This equivalence relation is in turn based onanother relation defined over the set of conflicting aircraft 16, namelythe conflict relation (‘A belongs to the same conflicting pair as B’),which establishes that an aircraft A1 is related to an aircraft A2 ifthey are in conflict with each other (or they are the same aircraft).The conflict relation is not an equivalence relation, as it does nothave the transitive property (if A1 is in conflict with A2 and A2 is inconflict with A3, A1 is not necessarily in conflict with A3). Theconflict dependency relation is based on the conflict relation asfollows: two aircraft 16 are considered related (equivalent) accordingto the conflict dependency relation if it is possible to connect them bymeans of a succession of conflict relations. It is easy to check thatthis relation fulfils the three properties of equivalence: reflexive,symmetric and transitive.

As an example, let us consider an aircraft A1 anticipated to enter inconflict with two different aircraft, A2 and A3, during a certainsegment of its trajectory. In addition, let us assume that A3 will alsocome in conflict with another aircraft, A4. As a result, the followingconflicts (conflict relations) will take place: A1-A2, A1-A3 and A3-A4.From these conflict relations it can immediately be seen that A1 isequivalent to A2 and to A3 and that A3 is equivalent to A4. In addition,by the transitive property A2 is equivalent to A3 (applying the conflictdependency relation: A2 is in conflict with A1, which is in conflictwith A3), A1 is equivalent to A4 (applying the conflict dependencyrelation: A1 is in conflict with A3, which is in conflict with A4) andA2 is equivalent to A4 (applying the conflict dependency relation: A2 isin conflict with A1, which is in conflict with A3, which is in conflictwith A4). Thus, the four aircraft 16 belong to the same equivalenceclass. The elements of an equivalence class are equivalent, under theequivalence relation, to all the others elements of the same equivalenceclass. Any two different equivalence classes in a non-empty set aredisjoint and the union over all of the equivalence classes is the givenset.

In the present context, the equivalence classes defined by the conflictdependency equivalence relation are the conflict dependency networksmentioned previously. It will now be understood that the aircraft 16belonging to each conflict dependent network are interconnected throughconflict dependency relations. Considering the properties of equivalencerelations, conflict dependent networks are disjoint, i.e. two aircraft16 cannot belong to two conflict dependent networks simultaneously. Inthe example above, A1, A2, A3 and A4 form a conflict dependent network.

Considering the above, the conflict detection process 110 first groupsthe conflicting aircraft 16 into conflict dependent networks at 424(using the information in the conflict list), and then groups theconflicts between the aircraft 16 in each conflict dependent networkinto a conflict sub-list. The conflict list contains as many sub-listsas there are conflict dependent networks. Analogously to the conflictdependent networks, conflict sub-lists are disjoint and their union isthe conflict list. Finally, the conflict detection process 110 ordersthe conflicts in each sub-list chronologically (earlier conflicts first)based on the first time step at which the applicable minimum is firstviolated (the start of the conflict interval).

Conflict Resolution

Completion of the conflict detection process 110 causes the conflictresolution process 120 to be called. The conflict detection process 110provides the conflict resolution process 120 with the conflict listorganized as a set of conflict sub-lists, each corresponding to aconflict dependent network.

The conflict resolution process 120 modifies the current aircraft intentdata 28 of at least some of the conflicting aircraft 16 so that theresulting trajectories are predicted to remain conflict-free and asefficient as possible. The conflict resolution process 120 only altersthe aircraft intent data 28 of the unlocked aircraft 16 in the conflictlist. Thus, it is assumed that there can be no conflicts involving onlylocked aircraft (these conflicts would have been resolved in a previousiteration of the conflict detection and resolution processes).

The conflict resolution process 120, for example in the case of arrivalmanagement, measures efficiency on the basis of predicted RunwayThreshold Crossing Time (tRT) for the aircraft 16. In particular, theobjective of the conflict resolution process 120 is to alter theaircraft intent data 28 in such a way that the resulting estimated valueof tRT deviates the least possible from the value that would be obtainedwith the user-preferred aircraft intent data 28 a.

The conflict resolution process 120 operates in a network-wise manner,attempting to get the aircraft 16 belonging to the same conflictdependent network to share equally the delays incurred in resolving theconflicts in which they are involved.

Let us assume that the conflict detection aircraft list 405 contains naircraft grouped into m disjoint conflict dependent networks. Let us nowconsider the conflict dependency network CDN_(j)={A₁ ^(j), . . . , A_(i)^(j), . . . , A_(n) _(j) ^(j)}, with

${i \in \left\{ {1,\ldots,n_{j}} \right\rbrack},{{j \in {\left\{ {1,\ldots,m_{j}} \right\} \mspace{14mu} {and}\mspace{14mu} \underset{j}{\Sigma}n_{j}}} = {n.}}$

All the conflicts in which an aircraft A_(j) ^(i)εCDN_(j) is involvedare contained in the conflict sub-list associated to CDNj, denoted asSLj. A Candidate Resolution Strategy (CRS) for an aircraft A_(i)^(j)εCDN is an instance of aircraft intent that, if implemented by A_(i)^(j) could potentially result in a conflict-free trajectory for theaircraft 16. In principle, any feasible aircraft intent for A_(i) ^(j)that is operationally meaningful in the scenario considered could beconsidered a candidate resolution strategy for that aircraft 16(including its preferred aircraft intent) since a conflict may beresolved as a result of actions. Candidate resolution strategies arederived from a set of predefined candidate resolution patterns (CRPs),which capture the allowable degrees of freedom that the aircraft 16 haveat its disposal to resolve conflicts in the scenario considered.Different CRPs target different conflict problems, for example someassist in an aircraft catching up and coming into conflict with anearlier aircraft and some assist in an aircraft falling behind intoconflict with a following aircraft. Selection of appropriate CRPs may bemade, as is described in more detail below.

A joint candidate resolution strategy (JCRS) for CDNj is a setcomprising of nj candidate resolution strategies, each assigned to oneof the aircraft in CDN_(j): JCRS_(j)={CRS₁ ^(j), . . . , CRS_(i) ^(j), .. . , CRS_(n) _(j) ^(j)} with JCRSj denoting a JCRS for CDNj and CRS_(i)^(j) denoting a candidate resolution strategy for the aircraft A_(i)^(j)εCDN_(j). A conflict-free JCRSj is a joint candidate resolutionstrategy for CDNj that is predicted to result in no conflicts involvingthe aircraft 16 in CDNj, i.e. SLj would become empty as a result ofimplementing a conflict-free JCRSj. To check whether a JCRSj isconflict-free, the conflict resolution process 120 must call theconflict detection process 110.

The objective of the conflict resolution process 120 is to design aconflict-free JCRSj that distributes the cost of resolving the conflictsin SLj among the aircraft belonging to CDNj in the most equitable waypossible.

It is assumed that the cost incurred by an aircraft A_(i) ^(j) a resultof implementing a strategy CRS_(i) ^(j) measured by the deviation thatCRS_(i) ^(j) causes from the aircraft operator's objectives (for thewhole trajectory or a segment). These objectives are captured by thetimeline corresponding to the trajectory that results from flyingaccording to the user-preferred aircraft intent and that is denoted ast_(RT) ^(pref). Thus, the cost of a candidate resolution strategyCRS_(i) ^(j) for A_(i) ^(j) is defined as follows:

c(CRS _(i) ^(j))=|t _(RT)(CRS _(i) ^(j))−t _(RT) ^(pref)|  (1)

where c(CRS_(i) ^(j)) is the cost of CRS_(i) ^(j), t_(RT)(CRS_(i) ^(j))is the arrival time for aircraft A_(i) ^(j) that is expected to resultfrom flying CRS_(i) ^(j).

As it stems from equation (1), the cost of delay and early arrival areconsidered to be the same. Thus, it is implicitly assumed that it is ascostly for the airline to arrive early as to arrive late. The costfunction could be adjusted to encode a higher cost of delay versus earlyarrival. For example, removing the absolute value from |t_(RT)(CRS_(i)^(j))−t_(RT) ^(pref)| in (1) would result in early arrivals having anegative cost, which would capture a situation where the airlineconsiders rewarding an early arrival.

Considering the above, the cost of a CRS measures the difference betweenthe arrival time that would result from flying the candidate resolutionstrategy and those that would result from flying the user-preferredaircraft intent, with the latter being the values preferred by theoperator. Thus, the cost of implementing the user-preferred aircraftintent as a CRS is zero, as it would result in no deviation from theuser-preferred arrival time.

In light of the above, the resolution of the conflicts in certainconflict sub-lists is cast as a constrained multi-objective optimizationproblem over the corresponding conflict dependent network. The problemis stated as follows:

minimise c(JCRS _(j))=(c(CRS _(i) ^(j)), . . . ,c(CRS _(i) ^(j)), . . .,c(CRS _(n) _(j) _(j)))

subject to JCRS _(j) εD _(j) ,D⊂X _(j)  (2)

where c(JCRS_(j)) is a vector function, with image in R^(n) ^(j) ,defined over the set Xj, which is the set of all possible jointcandidate resolution strategies for SLj. A vector c(JCRS_(j)) includesthe costs derived from each of the candidate resolution strategiescontained in JCRSj, a joint candidate resolution strategy for theaircraft 16 in CDNj. Dj denotes the set of conflict-free joint candidateresolution strategies for those aircraft 16. The solution) to theproblem in (2) would be one (or more) JCRS_(j)εD_(j) that simultaneouslyminimize, in some appropriate sense, the resolution costs as defined in(1) for all the aircraft 16 in the network.

It is not possible to define a single global optimum for a problem suchas the one in (2). Instead, as it is commonly done in multi-objectiveoptimization problems, we will assume that the solution consists of aset of acceptable trade-offs among the costs incurred by the aircraft16. The set of trade-offs considered is the Pareto set, which comprisesof all the Pareto-optimal solutions. A Pareto-optimal solution of (2) isa conflict-free JCRSj that is optimal in the sense that no otherconflict-free JCRSj can reduce the cost for an aircraft 16 in CDNjwithout increasing the cost for at least one other aircraft 16. Tocharacterize mathematically the Pareto set, it is necessary to extendthe relational operators =, ≦ and < to the set Z_(j)=Im(c(D_(j))), whichis the image of Dj on R^(n) ^(j) , i.e. Z_(j) ⊂R^(n) ^(j) . Thus,c(JCRS_(j))εZ_(j) ⊂R^(n) ^(j) . For any two vectors u, vεZ_(j), thefollowing relationships are defined:

u=v if ∀iγ{1, . . . ,n _(j) }:u _(i) =v _(i)

u≦v if ∀iε{1, . . . ,n _(j) }:u _(i) ≦v _(i)

u<v if u≦v and u≠v  (3)

Considering the definitions in (3), a conflict-free joint candidateresolution strategy JCRS*_(j) is said to be a Pareto-optimal solution tothe problem (2) if there is no JCRS_(j)εD such that

c(JCRS _(j))<c(JCRS* _(j))  (4)

The individual candidate resolution strategies that make up aPareto-optimal solution are denoted as CRS₁ ^(j)*, . . . , CRS_(i)^(j)*, . . . , CRS_(n) _(j) ^(j)*. Considering the individual costs inc(JCRS*_(j)) given by c₁(JCRS*_(i))=c(CRS₁ ^(j)*), . . . ,c_(i)(JCRS*_(i))=c(CRS_(i) ^(j)*), . . . , c_(n) _(j)(JCRS*_(i))=c(CRS_(i) _(n) _(j) ^(j)*), there is no JCRS_(j)εD_(j) thatcan cause a reduction in one of these costs without simultaneouslycausing an increase an at least one of the others. As said above, thePareto set of the problem (2), denoted as Pj, contains all theconflict-free joint candidate resolution strategies for CDNj that fulfil(4).

The conflict resolution process 120 proposes to resolve the conflicts inSLj by means of a JCRS*_(j) selected from the Pareto set, Pj. To thataim, the conflict resolution process 120 must first search forPareto-optimal solutions from which to choose. In other words, theconflict resolution process 120 must build a suitable subset of thePareto set. Once an appropriate number of conflict-free, Pareto-optimaljoint candidate resolution strategies have been found, the conflictresolution process 120 selects the one considered equitable according toaxiomatic bargaining principles. Axiomatic bargaining is a field of gametheory that provides axioms on how to select solutions with certainproperties, such as equity, to a game. In the present context, we canconsider the selection of the equitable JCRSj as a game involving theaircraft in CDNj. It is clear that an equitable solution to the gameshould be Pareto-optimal, JCRS*_(j), as a strategy that is notPareto-optimal will not be unanimously preferred by all players (it willnot be equitable to some players). However, Pareto-optimality alone isnot sufficient, as some Pareto-optimal solutions may be considered moreequitable than others. For example, some Pareto-optimal JCRS*_(j) mayresult in very high costs for some aircraft and very low costs for someother aircraft, while other Pareto-optimal JCRS*_(j) may distribute thecosts (i.e. time variation) among the aircraft 16 more equitably.Axiomatic bargaining principles will be used to guide the selection ofthe most equitable JCRS*_(j) among those found, with equity in thiscontext reflecting equality in cost distribution.

The selected most-equitable Pareto-optimal strategy is the one proposedto resolve the conflicts in SLj.

The mathematical method adopted to generate Pareto-optimal solutions to(2) is the linear weighting method, which consists of converting themulti-objective optimization problem into a single-objective one wherethe function to be minimized is a linear combination of the costs

C(C R S₁^(j)), …  , C(C R S_(i)^(j)), …  , C(C R S_(n_(j_(j)))^(j)).

The resulting single-objective minimization problem is stated asfollows:

minimise

w(JCRS _(j))=w ₁ c ₁(JCRS _(j))+ . . . +w _(i) c _(i)(JCRS _(j))+ . . .+w _(n) _(j) c _(n) _(j) (JCRS _(j))=w ₁ c(CRS ₁ ^(j))+ . . . +w _(i)c(CRS _(i) ^(j))+ . . . +w _(n) c(CRS _(n) _(j) ^(j))

subject to JCRS _(j) εD _(j) ,D _(j) ⊂X _(j)  (5)

The factors wi, with i ε{1, . . . , n}, are called weights and areassumed to be positive and normalized so that

${\underset{i}{\Sigma}w_{i}} = 1.$

Given a combination of values for the weights that comply with the aboveconditions, the solution of the resulting single-objective minimizationproblem (5) is a Pareto-optimal solution of the multi-objectiveminimization problem (2).

The problem of searching for an element of the Pareto set of (2) hasbeen recast as a constrained linear programming problem, which consistsof finding the global minimum of a single-objective constrainedminimization problem where the objective function is a linear functionof the costs associated to the individual candidate resolutionstrategies in a joint candidate resolution strategy.

The generation of candidate resolution strategies is at the core of theconflict resolution process 120. As mentioned above, the final aim ofthe conflict resolution process 120 is to find, for each conflictingaircraft 16, a candidate resolution strategy (i.e. an allowable instanceof aircraft intent) whose corresponding predicted trajectory is feasibleand conflict-free and results in an equitable share of the resolutioncosts for the operator. It has been seen that the search for anequitable, conflict-free joint candidate resolution strategy for aconflict dependent network is based on minimizing a function of thecosts associated to the individual candidate resolution strategies inthe joint candidate resolution strategy. Thus, the generation ofcandidate resolution strategies is at the core of the conflictresolution process 120.

The candidate resolution patterns (CRPs) mentioned above areparameterized instructions used as a template to generate differentinstructions of the same type. The amended instructions would result ina new trajectory that could resolve the conflicts in which the aircraft16 is involved. Examples of instructions that will be used to buildsimple candidate resolution patterns are:

Speed reduction: a sequence of instructions that result in a reducedaircraft speed. A speed reduction may be used to create a delay requiredto avoid coming into conflict with a preceding aircraft.

Speed increase: a sequence of instructions that result in an increasedaircraft speed. A speed increase may be used to gain time required toavoid coming into conflict with a following aircraft.

Altitude change: a sequence of instructions that result in an altitudechange.

Direct-to: a sequence of lateral instructions that result in a new RNAVhorizontal track where the aircraft 16 skips waypoints of the originalprocedure (it flies direct to a downstream waypoint). A direct-to may beused to create a delay or to avoid an area of conflict (see FIG. 9 a).

Path stretching: a sequence of lateral instructions that result in a newRNAV horizontal track where waypoints are added to the originalprocedure. Path stretching may be used to gain time or to avoid an areaof conflict (see FIG. 9 b).

When revising aircraft intent data 28 to remove a conflict, the natureof the conflict for the aircraft 16 currently being considered isdetermined. For example, whether the conflict arises because the currentaircraft 16 is catching up with the preceding aircraft 16 may bedetermined. If so, CRPs that create a delay may be selected.Alternatively, if the conflict arises because the current aircraft 16 isfalling behind and coming into conflict with a following aircraft 16,CRPs that give rise to gains in time may be selected. As a furtheralternative, conflicts arising from paths that cross rather thanconverge may see CRPs including an altitude change selected.

Once a CRP is selected, random changes to parameters of the aircraftintent 28 may be made, optionally within limits, to generate thecandidate resolution strategies. For example, random altitude changesmay be used, or random speed changes may be used. The candidateresolution strategies generated in this way for each aircraft 16 may begrouped into joint candidate resolution strategies and the best jointcandidate resolution strategies may be selected, as described above.

Considering the different concepts introduced above, there follows abrief step-by-step description of a full run of the conflict resolutionprocess 120, which is schematically explained by FIG. 10.

1. When a run of the conflict detection process 110 is completed at 701,the conflict detection process 110 calls the conflict resolution process120 at 702. The conflict detection process 110 provides the conflictresolution process 120 with the required conflict-related information,namely conflict dependent networks and conflict sub-lists.

2. The conflict resolution process 120 proceeds one conflict dependentnetwork at a time starting at 703, simultaneously considering all theconflicts in a sub-list.

3. For any network CDNj, the resolution of the conflicts in SLj is basedon a set of joint candidate resolution Patterns (JCRPs) for CDNj. AJCRPj is a JCRSj made up of candidate resolution patterns,JCRP_(j)={CRP₁ ^(j), . . . , CRP_(i) ^(j), . . . , CRP_(n) _(j) ^(j)}.To generate a JCRPj at 704, a candidate resolution pattern must beassigned to each of the aircraft in CDNj. In principle, any allowablecandidate resolution pattern for A_(i) ^(j) could be selected as CRP_(i)^(j). The only restriction on the candidate resolution patterns in JCRPjcomes from the fact that, when a conflict involves two aircraft with noearlier conflicts in SLj, at least one of the two aircraft must act uponthe conflict. Consequently, the candidate resolution pattern assigned toat least one of the two aircraft must include an alternative sequence ofinstructions that changes the aircraft intent and trajectory prior tothe initiation of the conflict interval (the sequence must be triggeredbefore the conflict starts). A series of heuristics will be in place toguide the selection of allowable candidate resolution patterns for A_(i)^(j) and the definition of the parameters and trigger conditions of thealternative sequences included in the selected candidate resolutionpatterns, as described above. These heuristics will be based on thepreferred intent of A_(i) ^(j) and the attributes of the conflicts inwhich it is involved. For example, the position of the conflict intervalalong the prediction timeline will help determine the triggers of thealternative instructions and the intensity and duration of the conflictswill help define the values of their parameters.

4. At 705, the conflict detection process 110 is called by the conflictresolution process 120 to check whether the generated JCRPjs areconflict-free. If no conflict-free JCRPjs can be found at 706, heuristicmethods are employed at 707 to extend CDNj by including the aircraft 16interfering with the JCRPjs. Thus, it is implicitly assumed that thereason why the allowable joint conflict resolution patterns do notresult in a conflict-free conflict dependent network is because theycreate conflicts with aircraft 16 outside the network. If an interferingaircraft 16 is itself including in a conflict dependent network, thenthat conflict dependent network must be considered in combination withCDNj for conflict resolution.

5. The resulting conflict-free JCRPjs are considered as the initialJCRSjs to initiate the search for Pareto-optimal conflict-freeJCRS*_(j)s at 708.

6. A subset of the Pareto set, i.e. set of conflict-free JCRS*_(j)s isbuilt at 709. To generate this subset, the minimization problem in (6)must be repeatedly solved for different sets of values for the weights,so as to obtain Pareto-optimal solutions that cover all areas of thePareto set. To resolve the minimization problem, a stochasticoptimization algorithm is employed. This algorithms will search for theminimum of w(JCRSj) from among JCRSjs generated from the initial jointconflict resolution patterns by randomizing the parameters and triggerconditions of the alternative instructions introduced in the CRP_(i)^(j)s.

7. Once a set of conflict-free Pareto-optimal solutions JCRS*_(j)s isavailable, the most equitable solution among the ones obtained isselected at 710 as the joint resolution strategy for CDNj, denoted asJRSj.

8. Steps 3 to 7 are performed for each of the identified conflictdependent networks. The Joint Resolution Strategy for all theconflicting aircraft is the combination of the JRSjs obtained for thedifferent CDNjs.

Variations

It will be clear to the skilled person that modifications may be made tothe embodiments described above without departing from the scope of thedisclosure.

For example, the present disclosure enjoys particular benefit whenapplied to air traffic management dealing with the most challengingscenario of predominantly converging paths such as terminal arrivals.Nonetheless, the present disclosure will of course also bring benefitsto less challenging environments like diverging paths as for terminaldepartures and also crossing paths.

It will be appreciated that the location of parts of the presentdisclosure may be varied. For example, trajectories may be calculated byground-based or air-based systems. For example, the air trafficmanagement may be ground-based, but need not necessarily be so. The airtraffic management need not be centralized. For example, a distributedair-based system could be possible.

Different air traffic management may cooperate and share information.For example, air traffic management having responsibility for adjacentairspaces may pass trajectory information for aircraft anticipated tocross between the adjacent airspaces.

1. A computer-implemented method of managing airspace through which aplurality of aircraft are flying, comprising: receiving, from theaircraft, user preferred aircraft intent data that unambiguously definesthe user preferred trajectory of each aircraft; calling an initialglobal conflict detection procedure comprising: calculating thecorresponding user preferred trajectories from the user preferredaircraft intent data; and comparing the user preferred trajectories soas to identify one or more conflicts between trajectories; using thecomparison of the user preferred trajectories to identify conflictedaircraft and to place the conflicted aircraft into conflict dependentnetworks such that each conflict dependent network contains a set of allaircraft in conflict with each other, either directly or indirectly,such that each conflicted aircraft can be a member of one conflictdependent network only; for each conflict dependent network, calling aninitial conflict resolution procedure comprising: revising the userpreferred aircraft intent data of one or more of the conflicted aircraftof the conflict dependent network to produce revised aircraft intentdata defining a corresponding revised trajectory in a way that shouldremove conflicts from within that conflict dependent network; andsending the revised aircraft intent data to the corresponding conflictedaircraft.
 2. The method of claim 1 further comprising, for each conflictdependent network, calling a local conflict detection procedure whereinthe local conflict detection procedure comprises: calculating thecorresponding revised trajectories from the revised aircraft intentdata; and comparing the user-preferred trajectories from the aircraft inthe conflict dependent network not subject to revised aircraft intentdata and revised trajectories from the aircraft in the conflictdependent network subject to revised aircraft intent data so as toidentify one or more conflicts and to identify the still-conflictedaircraft and, if conflicts are identified, calling a further conflictresolution procedure, wherein the further conflict resolution procedurecomprises revising the user-preferred aircraft intent data of one ormore of the still-conflicted aircraft of the conflict dependent networkto produce revised aircraft intent data in a way that should removeconflicts from within that conflict dependent network, and calling afurther local conflict detection procedure, or if conflicts are notidentified, continuing to process the next conflict dependent networkor, if all conflict dependent networks have been processed, calling afurther global conflict detection process; wherein the further globalconflict detection process comprises: calculating the correspondingrevised trajectories from the revised aircraft intent data; comparingthe user-preferred trajectories from the aircraft not subject to revisedaircraft intent data and revised trajectories from the aircraft subjectto revised aircraft intent data so as to identify one or more conflictsbetween trajectories; and if conflicts are detected, calling the localconflict resolution procedure, optionally after merging separateconflict dependent networks where a conflict arises between aircraft inthe separate conflict dependent networks, or if no conflicts aredetected, continuing to the step of sending the revised aircraft intentdata to the corresponding conflicted aircraft.
 3. The method of claim 1further comprising, for each conflict dependent network, calling a localconflict detection procedure, wherein the local conflict detectionprocedure comprises: calculating the corresponding revised trajectoriesfrom the revised aircraft intent data; and comparing the user-preferredtrajectories from the aircraft in the conflict dependent network notsubject to revised aircraft intent data and revised trajectories fromthe aircraft in the conflict dependent network subject to revisedaircraft intent data so as to identify one or more conflicts and toidentify the still-conflicted aircraft and, if conflicts are identified,calling a further conflict resolution procedure, wherein the furtherconflict resolution procedure comprises revising the user-preferredaircraft intent data of one or more of the still-conflicted aircraft ofthe conflict dependent network to produce revised aircraft intent datain a way that should remove conflicts from within that conflictdependent network, and continuing to process the next conflict dependentnetwork or, if all conflict dependent networks have been processed,calling a further global conflict detection process, or if conflicts arenot identified, continuing to process the next conflict dependentnetwork or, if all conflict dependent networks have been processed,calling a further global conflict detection process; wherein the furtherglobal conflict detection process comprises: calculating thecorresponding revised trajectories from the revised aircraft intentdata; comparing the user-preferred trajectories from the aircraft notsubject to revised aircraft intent data and revised trajectories fromthe aircraft subject to revised aircraft intent data so as to identifyone or more conflicts between trajectories; and if conflicts aredetected, calling the local conflict resolution procedure, optionallyafter merging separate conflict dependent networks where a conflictarises between aircraft in the separate conflict dependent networks, orif no conflicts are detected, continuing to the step of sending therevised aircraft intent data to the corresponding conflicted aircraft.4. The method of claim 2, wherein performing the initial conflictresolution procedure and any further conflict resolution procedurescomprise processing the conflict dependent networks in turn inincreasing order of the lateness of the earliest occurring conflictwithin the conflict dependent network.
 5. The method of claim 4, whereinperforming the initial conflict resolution procedure and any furtherconflict resolution procedures comprises selecting the earliestoccurring conflict in the conflict dependent network and amending theaircraft intent data of one of the aircraft involved in that conflict,selecting the next earliest occurring conflict in the conflict dependentnetwork and amending the aircraft intent data of one of the aircraftinvolved in the that conflict, and so on until all conflicts have beenselected.
 6. The method of claim 2 wherein the step of calling theinitial conflict resolution procedure or any step of calling the furtherconflict resolution procedure comprises, for each conflict dependentnetwork: selecting one or more identified conflicts from the conflictdependent network, characterising the selected conflict, andresponsively revising the user-preferred aircraft intent data or therevised aircraft intent data of one of the aircraft involved in theselected conflict in a way that should remove the conflict.
 7. Themethod of claim 6, wherein the step of responsively revising theuser-preferred aircraft intent data or revised aircraft intent data isperformed by selecting a revision from a set of candidate resolutionpatterns.
 8. The method of claim 7, wherein the step of responsivelyrevising the user-preferred aircraft intent data or revised aircraftintent data further comprises: storing each instance of revised aircraftintent data so as to form a first joint candidate resolution patternwhen all conflicts have been removed; repeating the steps of calling theinitial conflict detection and resolution procedures and, if conflictsare found, calling the further conflict detection procedure so as toform at least a second joint candidate resolution pattern; selecting oneof the joint candidate resolution strategies; and sending the revisedaircraft intent data from the selected joint candidate resolutionstrategy to the corresponding conflicted aircraft.
 9. The method ofclaim 8, comprising selecting one of the joint candidate resolutionstrategies according to an evaluation of the total change in path lengthor time of arrival of the revised trajectories for each joint candidateresolution strategy.
 10. The method of claim 8, comprising selecting oneof the joint candidate resolution strategies according to an evaluationof how each joint candidate resolution strategy distributes changes intrajectories between the aircraft or changes in time of arrival betweenthe aircraft.
 11. The method of claim 7, wherein the step ofcharacterising the selected conflict in the initial or further conflictresolution procedure comprises determining the cause of the reducedseparation between the conflicted aircraft, and revising the aircraftintent data comprises revising aircraft intent data to reverse thecause.
 12. The method of claim 11, wherein the candidate resolutionpatterns include patterns that cause the aircraft intent data to berevised to cause at least one of: an increase in altitude of one of theconflicted aircraft, a decrease in altitude of one of the conflictedaircraft, an increase in speed of one of the conflicted aircraft, adecrease in speed of one of the conflicted aircraft, an increase in pathlength of one of the conflicted aircraft optionally by adding one ormore waypoints to the path, and a decrease in path length of one of theconflicted aircraft optionally by removing one or more waypoints fromthe path.
 13. The method of claim 12, wherein the step of responsivelyrevising the user preferred aircraft intent data or revised aircraftintent data in the initial or further conflict resolution procedure isperformed in a partially random manner.
 14. The method of claim 13,wherein selecting a revision from a set of candidate resolution patternscomprises determining which of the patterns are suitable for removingthe selected conflict, and selecting randomly one of the suitablepatterns.
 15. The method of claim 13, wherein selecting a revision froma set of candidate resolution patterns comprises determining which ofthe patterns are suitable for removing the selected conflict, selectingone of the suitable patterns and revising a parameter associated withthe selected pattern by a random amount. 16-19. (canceled)
 20. A systemfor managing airspace through which a plurality of aircraft are flying,the system comprising: a computer apparatus; a non-transitory computerreadable medium comprising instructions stored thereon, that whenexecuted by the computer apparatus, causes the computer apparatus to:receive, from the aircraft, user preferred aircraft intent data thatunambiguously defines the user preferred trajectory of each aircraft;call an initial global conflict detection procedure comprising:calculating the corresponding user preferred trajectories from the userpreferred aircraft intent data; and comparing the user preferredtrajectories so as to identify one or more conflicts betweentrajectories; use the comparison of the user preferred trajectories toidentify conflicted aircraft and to place the conflicted aircraft intoconflict dependent networks such that each conflict dependent networkcontains a set of all aircraft in conflict with each other, eitherdirectly or indirectly, such that each conflicted aircraft can be amember of one conflict dependent network only; for each conflictdependent network, calling an initial conflict resolution procedurecomprising: revising the user preferred aircraft intent data of one ormore of the conflicted aircraft of the conflict dependent network toproduce revised aircraft intent data defining a corresponding revisedtrajectory in a way that should remove conflicts from within thatconflict dependent network; and send the revised aircraft intent data tothe corresponding conflicted aircraft.
 21. The system of claim 20wherein said non-transitory computer readable medium comprises furtherinstructions, that when executed by the computer apparatus, causes thecomputer apparatus to: for each conflict dependent network, call a localconflict detection procedure wherein the local conflict detectionprocedure comprises: calculating the corresponding revised trajectoriesfrom the revised aircraft intent data; and comparing the user-preferredtrajectories from the aircraft in the conflict dependent network notsubject to revised aircraft intent data and revised trajectories fromthe aircraft in the conflict dependent network subject to revisedaircraft intent data so as to identify one or more conflicts and toidentify the still-conflicted aircraft and, if conflicts are identified,calling a further conflict resolution procedure, wherein the furtherconflict resolution procedure comprises revising the user-preferredaircraft intent data of one or more of the still-conflicted aircraft ofthe conflict dependent network to produce revised aircraft intent datain a way that should remove conflicts from within that conflictdependent network, and calling a further local conflict detectionprocedure, or if conflicts are not identified, continuing to process thenext conflict dependent network or, if all conflict dependent networkshave been processed, calling a further global conflict detectionprocess; wherein the further global conflict detection processcomprises: calculating the corresponding revised trajectories from therevised aircraft intent data; comparing the user-preferred trajectoriesfrom the aircraft not subject to revised aircraft intent data andrevised trajectories from the aircraft subject to revised aircraftintent data so as to identify one or more conflicts betweentrajectories; and if conflicts are detected, calling the local conflictresolution procedure, optionally after merging separate conflictdependent networks where a conflict arises between aircraft in theseparate conflict dependent networks, or if no conflicts are detected,continuing to the step of sending the revised aircraft intent data tothe corresponding conflicted aircraft.
 22. The system of claim 20wherein said non-transitory computer readable medium comprises furtherinstructions, that when executed by the computer apparatus, causes thecomputer apparatus to: for each conflict dependent network, call a localconflict detection procedure, wherein the local conflict detectionprocedure comprises: calculating the corresponding revised trajectoriesfrom the revised aircraft intent data; and comparing the user-preferredtrajectories from the aircraft in the conflict dependent network notsubject to revised aircraft intent data and revised trajectories fromthe aircraft in the conflict dependent network subject to revisedaircraft intent data so as to identify one or more conflicts and toidentify the still-conflicted aircraft and, if conflicts are identified,calling a further conflict resolution procedure, wherein the furtherconflict resolution procedure comprises revising the user-preferredaircraft intent data of one or more of the still-conflicted aircraft ofthe conflict dependent network to produce revised aircraft intent datain a way that should remove conflicts from within that conflictdependent network, and continuing to process the next conflict dependentnetwork or, if all conflict dependent networks have been processed,calling a further global conflict detection process, or if conflicts arenot identified, continuing to process the next conflict dependentnetwork or, if all conflict dependent networks have been processed,calling a further global conflict detection process; wherein the furtherglobal conflict detection process comprises: calculating thecorresponding revised trajectories from the revised aircraft intentdata; comparing the user-preferred trajectories from the aircraft notsubject to revised aircraft intent data and revised trajectories fromthe aircraft subject to revised aircraft intent data so as to identifyone or more conflicts between trajectories; and if conflicts aredetected, calling the local conflict resolution procedure, optionallyafter merging separate conflict dependent networks where a conflictarises between aircraft in the separate conflict dependent networks, orif no conflicts are detected, continuing to the step of sending therevised aircraft intent data to the corresponding conflicted aircraft.23. The system of claim 21 wherein performing the initial conflictresolution procedure and any further conflict resolution procedurescomprise processing the conflict dependent networks in turn inincreasing order of the lateness of the earliest occurring conflictwithin the conflict dependent network.
 24. A non-transitory computerreadable medium having stored thereon a computer program for managingairspace through which a plurality of aircraft are flying, the computerprogram comprising instructions that when executed by a computerapparatus causes the computer apparatus to: receive, from the aircraft,user preferred aircraft intent data that unambiguously defines the userpreferred trajectory of each aircraft; call an initial global conflictdetection procedure comprising: calculating the corresponding userpreferred trajectories from the user preferred aircraft intent data; andcomparing the user preferred trajectories so as to identify one or moreconflicts between trajectories; use the comparison of the user preferredtrajectories to identify conflicted aircraft and to place the conflictedaircraft into conflict dependent networks such that each conflictdependent network contains a set of all aircraft in conflict with eachother, either directly or indirectly, such that each conflicted aircraftcan be a member of one conflict dependent network only; for eachconflict dependent network, calling an initial conflict resolutionprocedure comprising: revising the user preferred aircraft intent dataof one or more of the conflicted aircraft of the conflict dependentnetwork to produce revised aircraft intent data defining a correspondingrevised trajectory in a way that should remove conflicts from withinthat conflict dependent network; and send the revised aircraft intentdata to the corresponding conflicted aircraft.